Melt reduction arc furnaces are used for the melt reduction of the iron oxide in iron ore in particular, but may be used for the melt reduction of other metal oxides.
Such a furnace is conventionally constructed substantially like an electric arc furnace used for making steel but uses an arcing electrode having a longitudinally extending passageway through which in powdered form the iron ore is fed to the working tip of the electrode. The furnace is powered by DC, the furnace hearth having means for electrically connecting the melt in the hearth to the DC power source to which the electrode is connected. The electrode is centrally positioned on the vertical axis of the furnace and if more than one such electrode is used, the electrodes are closely grouped about this axis. A highly carbonaceous iron melt is maintained in the furnace's hearth and its carbon reacts with the iron oxide of the fed ore with its consequent reduction to molten iron which continuously adds to the melt, the iron melt being withdrawn from the hearth as required to keep the melt level in the hearth from excessively increasing in height. A coke bed can be kept floating on the iron melt to maintain its carbon content and powdered carbonaceous material may be fed with the ore through the electrode or otherwise. This is a continuous process, the powdered iron oxide material being fed successively at intervals or continuously and the reaction between the iron oxide and the ore being therefore substantially continuous.
The reaction between the iron oxide and carbon continuously produces carbon monoxide gas and possibly hydrogen gas. The iron ore may be pretreated so that its iron oxide is partially reduced when fed to the furnace, but because the operation is continuous, the gases are produced in large volumes and must be continuously removed from the furnace. For this it is conventional for an electric arc furnace roof to be built with a gas outlet through which the gases are discharged.
For years an arc furnace roof has been dome-shaped and layed up from bricks with the centrally positioned electrode or electrodes using much of its area, and this has required its gas outlet to be of such small size that the gases produced by the melt reduction process pass through the outlet at such high velocity and temperature as to produce an undesirably rapid erosion of the outlet as well as of the roof brickwork, particularly around the outlet. Because the practice of the melt reduction process involves the use of a tubular electrode, normally of the graphite or Soderberg type, it is less resistant to breakage than the solid electrodes used for steel-making. For practical considerations, the periphery of the roof must be spaced a certain minimum above the hearth and its melt, and this height substantially increases at the center of the roof through which the electrode projects, because of the roof's dome shape. Consequently, the length of the tubular electrode inside of the furnace must be undesirably long considering its lowered resistance to breakage. The electrode burns off at its tip during operation, and must have a length permitting continual feeding of the electrode into the furnace. It is not unusual for the electrode to have a length in the order of 15 meters. It is expensive to make, and it is desirable to use the entire electrode length productively without loss due to breakage in the furnace.